Transmit-receive (Tx-Rx) switches are typically designed for frequency ranges up to around 2.5 GHz, for example when a switch is integrated on an ASIC. Discrete switches may be used at higher frequencies. An example from reference [1] of an integrated switch is illustrated in FIG. 1, which shows a transceiver 100 having a power amplifier (PA) 101 and a low noise amplifier (LNA) 102, both of which are connected to an antenna 103. The PA 101 is connected to the antenna 103 via a transformer 104 and a transmit (Tx) switch 105. A further transmit switch 106 is provided between an input of the LNA 102 and ground. When the Tx switches 105, 106 are activated, the output of the PA 101 is fed to the antenna 103 and the input of the LNA is grounded.
FIGS. 2 and 3 illustrate another approach from references [2] and [3], in which Tx-Rx switches employ two or more transformers (baluns), which may be integrated or provided as off-chip components. In the transceiver 200 illustrated in FIG. 2, the PA 201 and LNA 202 are switched using an external balun 203, while in the transceiver 300 illustrated in FIG. 3 the PA 301 and LNA 302 are switched using an internal impedance matching circuit 303.
In the first type of transceiver 100, i.e. that of FIG. 1, the series switch 105 between the PA 101 and the antenna 103 will tend to result in output power loss. In addition, the parasitic capacitance of the switch 105 will tend to limit the working frequency range of the transceiver 100. In the second approach, i.e. that of either FIG. 2 or FIG. 3, employing two or more transformers is considerably more expensive, especially in the case of using an off-chip transformer (in the FIG. 2 arrangement). In addition to manufacturing cost, designing two transformers for wideband applications is very difficult, which will tend to limit the use of this type of arrangement to narrowband applications.